Helicopter gas turbine engine systems mix ambient air with fuel to provide a combustible energy source for the gas turbine engine systems. Such engine systems perform more efficiently if the ambient air is provided under non-turbulent flow conditions. This is typically achieved by channeling ambient air into the forward portion of the engine system through an engine inlet duct that is configured to maintain the intake airflow in a non-turbulent condition. Turbulence may be induced in the intake airflow, however, due to structural discontinuities at the attachment or integration zones of the engine inlet duct.
For externally mounted engine systems, for example, the aft end of the engine inlet duct is integrated in combination with the engine system and the forward end of the engine inlet duct is directly exposed to the flight environment to channel ambient air into the engine system via the engine inlet duct. For internally mounted engine systems, in contrast, the aft end of the engine inlet duct is integrated in combination with the engine system internally within the airframe of the aircraft and the forward end of the engine inlet duct is generally integrated in combination with the airframe. Ambient airflow over the airframe is diverted to the engine by means of the engine inlet duct.
Structural discontinuities may occur at the forward and/or aft integration zones which will induce turbulence in the intake airflow. Such structural discontinuities may arise as a result of manufacturing variations in the fabrication of the engine inlet duct which may cause structural discontinuities when the engine inlet duct is integrated in combination with the engine system and/or airframe. In addition, aperiodic transient structural discontinuities may occur as a result of relative motion between the engine inlet duct and the engine system during helicopter operations.
Since the engine inlet duct for internally mounted engines is effectively fixed at both the forward and aft ends thereof and is enclosed within the airframe, the fabrication of internal engine inlet ducts requires close manufacturing tolerances to ensure proper integration of the engine inlet duct with the airframe and the engine system, i.e., to preclude any structural discontinuities. In addition, structural discontinuities may arise as a result of relative motion between the engine inlet system and the engine system. Such structural discontinuities may be especially problematical in helicopters where the engine system is structurally and functionally interconnected with the main transmission gearbox and subject to deflections induced by the main transmission gearbox. Such deflections are a result of the torque loads of the gearbox and/or applied loads due to aircraft flight maneuvers.
Out-of-tolerance inlet engine duct structures, which preclude proper integration, and/or relative motion between the engine inlet duct and the engine system due to induced deflections, may cause structural discontinuities at the integration zone of the engine inlet duct and the engine system. Such structural discontinuities create turbulence in the channeled intake airflow provided to the engine system by means of the engine inlet duct. The operation of the engine system may be adversely affected to varying degrees depending upon the magnitude of the turbulence induced in the intake airflow in the engine inlet duct due to such structural discontinuities. For example, highly turbulent intake airflow causes the compressor of gas turbine engine systems to effectively "see" shock waves which cause compressor stall.
A need exists to provide a flexible engine inlet duct mounting system for helicopter engine systems that accommodates manufacturing deviations in the engine inlet duct wherein the engine inlet duct may be properly integrated in combination with the helicopter airframe and the engine system so that structural discontinuities are eliminated. In addition, the mounting system should accommodate transient, aperiodic relative motion between the helicopter airframe, the main transmission gearbox, and the internal engine system without causing turbulence-inducing structural discontinuities at the integration zone between the engine inlet duct and the engine system.